TPS61170 Datasheet, PDF (8/22 Pages) Texas Instruments – 1.2A High Voltage Boost Converter in 2x2mm2 QFN Package

TPS61170
SLVS789 – NOVEMBER 2007
DETAILED DESCRIPTION
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OPERATION
The TPS61170 integrates a 40-V low side FET for up to 38-V output voltages. The device regulates the output
with current mode PWM (pulse width modulation) control. The switching frequency of PWM is fixed at 1.2MHz.
The PWM control circuitry turns on the switch at the beginning of each switching cycle. The input voltage is
applied across the inductor and stores the energy as inductor current ramps up. During this portion of the
switching cycle, the load current is provided by the output capacitor. When the inductor current rises to the
threshold set by the error amplifier output, the power switch turns off and the external Schottky diode is forward
biased. The inductor transfers stored energy to replenish the output capacitor and supply the load current. This
operation repeats in every switching cycle. As shown in the block diagram, the duty cycle of the converter is
determined by the PWM control comparator which compares the error amplifier output and the current signal.
A ramp signal from oscillator is added to the current ramp. This slope compensation is to avoid sub-harmonic
oscillation that is intrinsic to the current mode control at duty cycle higher than 50%. The feedback loop regulates
the FB pin to a reference voltage through an error amplifier. The output of the error amplifier is connected to the
COMP pin. An external RC compensation network is connected to the COMP pin to optimize the feedback loop
for stability and transient response.
SOFT START-UP
Soft-start circuitry is integrated into the IC to avoid a high inrush current during start-up. After the device is
enabled by a logic high signal on the CTRL pin, the FB pin reference voltage ramps up in 32 steps, each step
takes 213µs. This ensures that the output voltage rises slowly to reduce inrush current. Additionally, for the first
5msec after the COMP voltage ramps, the current limit of the PWM switch is set to half of the normal current limit
spec. Therefore, during this period the input current is kept below 700mA (typical). See the start-up waveform for
a typical example, Figure 11.
OVERCURRENT PROTECTION
TPS61170 has a cycle-by-cycle overcurrent limit feature that turns off the power switch once the inductor current
reaches the overcurrent limit. The PWM circuitry resets itself at the beginning of the next switch cycle. During an
over-current event, this results in a decrease of output voltage with respect to load. The current limit threshold as
well as input voltage, output voltage, switching frequency and inductor value determine the maximum available
output current. Larger inductor values increases the current output capability because of the reduced current
ripple. See the APPLICATION INFORMATION section for the output current calculation.
UNDERVOLTAGE LOCKOUT (UVLO)
An undervoltage lockout prevents mis-operation of the device at input voltages below typical 2.2V. When the
input voltage is below the undervoltage threshold, the device remains off and the internal switch FET is turned
off. The undervoltage lockout threshold is set below minimum operating voltage of 3V to avoid any transient VIN
dip triggering the UVLO and causing the device to reset. For the input voltages between UVLO threshold and 3V,
the device maintains its operation, but the specifications are not ensured.
THERMAL SHUTDOWN
An internal thermal shutdown turns off the device when the typical junction temperature of 160°C is exceeded.
The IC restarts when the junction temperature drops by 15°C.
ENABLE AND SHUTDOWN
The TPS61170 enters shutdown when the CTRL voltage is less than 0.4V for more than 2.5ms. In shutdown, the
input supply current for the device is less than 1µA (max). The CTRL pin has an internal 800kΩ pull down
resistor to disable the device when the pin is left unconnected.
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